This invention relates to fracture and abrasion resistant materials; more particularly, it is concerned with a process for the production of silicon nitride based ceramic bodies exhibiting improved fracture toughness and strength.
In pursuing a goal of development of new, tougher ceramics for use in demanding, high temperature structural applications, numerous attempts have been made to toughen silicon nitride by the dispersion of particles, whiskers, or fibers in ceramics as reinforcing material. In certain cases these composites exhibit modest improvement in strength along with an increase in toughness, but more often any increase in toughness is achieved at the expense of a reduction in strength.
The present invention is directed to a process for producing composite silicon nitride bodies of a bimodal composite material including whiskers or fibers and a blend of fine and coarse silicon nitride powders. The process consistently yields a material of high fracture toughness and strength above those of state-of-the-art materials, and well above those of composite materials made from either the fine or the coarse silicon nitride materials alone. This achievement of improved fracture toughness and strength is unexpected, since Ube Industries, Ltd. of Tokyo, Japan have reported the sintering of a binary powder mix to form a monolithic silicon nitride body which exhibits fracture toughness and strength, e.g. at 20 percent by weight fine silicon nitride powder of average diameter about 0.18 .mu.m and 80 percent by weight 1.89 .mu.m coarse powder, lower than those of a similar material containing 100 percent fine powder.